This invention relates to treatment of vascular obstruction.
A fixed obstruction within a blood vessel, for example, an atherosclerotic plaque or an atheroembolus, can reduce blood flow to a level inadequate to provide for the metabolic demand of tissues supplied by the blood vessel. Vasospasm of an artery can produce similar problems because of partial or complete obstruction of an artery which otherwise appears normal. The combination of such dynamic or spastic constriction at the point of a clinically benign fixed obstruction can create a clinically severe reduction of flow. Vasospasm can occur spontaneously, or because of environmental stimuli, including either pharmacologic stimuli, for example, ergonovine testing, and mechanical stimuli, e.g., contact with a surgical instrument, or a diagnostic or therapeutic catheter as a complication of percutaneous transluminal catheter angioplasty.
Approaches proposed for treating vascular obstruction include inducing or causing vasodilation. They also include removing or reducing the size of a fixed obstruction to partially or fully relieve the obstruction and permit restoration of a sufficient blood flow. Examples of these approaches include pharmacotherapeutic treatments and mechanical procedures. J. B. Young et al., "Medical Options in Chronic Stable Angina," Cardiovascular Medicine, July, 1985, pp. 21-30.
Pharmacotherapeutic treatments for angina include administration of nitrates. These dilate peripheral vessels and reduce resistance in the coronary circulation. Nitrates, however, are not always effective, and some people suffer intolerable side effects.
Mechanical procedures for relieving stenoses in which an occlusive lesion, e.g., an atheroslerotic plaque, is present include bypass surgery and percutaneous transluminal angioplasty (PTA). PTA includes balloon angioplasty, where the plaque and intima of the vessel are mechanically displaced by inflation of a balloon. The balloon is positioned in the lumen of the vessel at the stenosis. Not all patients can benefit from balloon angioplasty.
The mechanical procedures also include laser thermal angioplasty, where plaque formations are ablated by irradiation with laser energy. Laser thermal angioplasty is an inappropriate treatment for vasospasm where no fixed obstruction is present to be ablated. Moreover, severe vasospasm, refractory to pharmaceutical dilators, can be a complication of continuous wave laser thermal angioplasty.
An example of use of the combination of a dilation catheter and a laser is provided by Littleford, U.S. Pat. No. 4,834,093. A laser is used to ablate tissue so that a balloon catheter can be subsequently inserted within the remaining tissue and inflated to displace the vessel tissue.
DovGal et al., Abstract 2085 of the 60th Scientific Sessions, Circulation 1987, describe use of a pulsed laser (at 12 MJ/pulse, 10-80 Hz) rather than a continuous laser in order to avoid problems of vasoconstriction or vasospasm.